1. Field of the Invention
The invention relates to a display screen with matrix access using the mixed thermal and electric effect and displaying an image or information in color.
2. Description of the Prior Art
It is known to use a composite thermal and electric effect with smectic liquid crystals for forming screens of this type. They may be used either for projection or for direct vision.
Three distinct devices may for example be used each corresponding to a primary color and the images of which are projected with superimposition on a screen so as to restore the original image. The drawback is the large amount of space occupied and the complexity.
Another means of obtaining a color image consists in inserting, in the optical path between an illumination source and a screen, a filter formed of parallel colored strips and combined optically with a display cell. This system only uses a single cell but also has the disadvantage of taking up a lot of space and having to dispose certain elements at well defined distances. It forms the object of a French patent application belonging to the applicant under the national number 79.23.599 and filed on the Sept. 21, 1979.
It is further known (French patent of the applicant filed on the May 5, 1977 under the national number 77.13 738) to obtain a color image from a display cell of the type comprising a layer of material which may be written on by thermal-electric effect, in which dielectric mirrors, alternately red, green and blue, are placed above heating lines and parallel thereto. One of the drawbacks of this device resides in the difficulty of forming these mirrors.
Another method for obtaining one color display device consists in using dichroic coloring matters in solution in a nematic liquid crystal. By using the appropriate electric field, the molecules of the liquid crystal and the dichroic molecules may be slanted to go over from an absorbing state to a non absorbing state. Thus a variation in the orientation of the molecules is transformed into an absorption variation. The elementary point is therefore either transparent or colored, having the color of the diochroic coloring matter dissolved in the liquid crystal. The practical problem which occurs with dichroic coloring matter devices is to find a stable coloring molecule which has a sufficiently good parameter of angular order.